CDK4 and breast carcinoma: To date, three CDK4/6 inhibitors (palbociclib, ribociclib, and abemaciclib) have received regulatory approval for the treatment of HR+/HER– breast cancer in combination with aromatase inhibitors (AIs) or fulvestrant, or as a monotherapy (abemaciclib only) in adjuvant and metastatic treatment settings [5,6,7].